Left Ventricular Assist Devices (LVADs) are auxiliary pouches intended to function as booster pumps to aid the hearts of individuals suffering from chronic congestive heart failure. This condition is frequently due to heart attacks that reduce the pumping capacity of the human heart. By boosting the capacity of such a weakened heart, individuals suffering from this condition may be allowed to again lead relatively normal, effective lives.
While various designs of LVADs have been proposed, the most promising appears to be an auxiliary pouch formed from the individual's latissimus dorsi muscle and controlled by a pacemaker. This approach avoids potential rejection problems related to the use of other non-autologous materials and takes advantage of well-developed pacemaker and prosthetic vascular graft technology. LVADs of this type are commonly called skeletal muscle ventricles (SMVs). Much of the developmental work on these devices has been accomplished by Dr. Larry Stephenson and colleagues at Wayne State University. Their work has been described in various articles in the literature; see, for example, 1) Clark M, Springen K, Help For The Heart: Back Muscle. Newsweek Dec. 22, 1986. 2) Mannion JD et al., Hydraulic pouches of canine latissimus dorsi: Potential for left ventricular assistance. Journal of Thoracic Cardiovascular Surgery 1986; 91:534-544. 3) U.S. Pat. No. 4,979,936. 4) Thomas GA et al. Pericardium-Lined Skeletal Muscle Ventricles in Circulation up to 589 Days. Society of Thoracic Surgeons 1994; 58:1-11.
Based on development work done previously on beagles, it is anticipated that the procedure for creating such an SMV in a human would involve making an incision to expose the left latissimus dorsi muscle and dissecting the muscle free from the subcutaneous tissues and chest wall, except for the neurovascular bundle and humeral insertion. A bipolar nerve cuff electrode is placed around the thoracodorsal nerve. The nerve lead is connected to an inactive neurostimulator, buried beneath the left rectus abdominis muscle, which innervates the exposed latissimus dorsi muscle.
Next, the left chest is opened at the fourth rib. Preferably, the fourth rib is removed to provide more space for the LVAD. Optionally, the anterior pericardium is removed between the phrenic nerves and used to cover a conically-shaped mandrel of biocompatible plastic. Mandrels used for beagles had a diameter of about 3 cm, length of about 6.5 cm and volume of about 25 ml; a mandrel suitable for forming a human SMV would need to be appropriately enlarged. After wrapping the pericardium around the mandrel it is sewn to a 5 mm thick collar of synthetic material such as woven Dacron felt placed at the base of the mandrel. The dorsal edge of the latissimus dorsi muscle is then folded longitudinally upon itself and secured by sutures, after which the medial aspect of the latissimus dorsi muscle is wrapped around the mandrel (and over the pericardium if it was used) about 2-2.5 times with the folded edge of the muscle sewn circumferentially to the Dacron felt collar. The SMV is then positioned subcutaneously and the wound is closed and allowed to heal for three weeks.
Following this healing period, the neurostimulator is activated to deliver continuously a 2 Hz stimulation of 210 microsecond duration and 1 to 2 volt amplitude. The purpose of the stimulation is to transform the fatigable Type II latissimus dorsi muscle fibers to fatigue-resistant Type I muscle fibers. Typically, 6 weeks are allowed for this stimulation period, after which the chest is again opened to connect the formed muscle pouch to the aorta. This is accomplished by first attaching sensing leads to the left ventricle. The descending thoracic aorta is exposed to allow two 12 mm ringed vascular grafts to be anastomosed to the aorta, one above the other, in end-to-side fashion.
After completion of these anastomoses, a cotton tape is passed around the aorta between the two graft anastomoses for subsequent ligation of the aorta. Next, a circular 3.5 cm diameter piece of patch material is cut from a sheet of 0.6 mm thick GORE-TEX.RTM. Cardiovascular Patch. This circular piece may optionally be formed into a concave, cup-like shape. A pair of 12 mm diameter holes are cut through this sheet to accommodate anastomosis of the opposite ends of the two vascular grafts. The plastic mandrel is removed from within the muscle pouch. The Dacron felt collar remains. The 3.5 cm diameter circular piece of GORE-TEX Cardiovascular Patch is sewn over the open end of the muscle pouch (with the concave side facing the pouch if the patch was so formed) and the opposite ends of the two vascular grafts are anastomosed to the 12 mm diameter holes in the patch. The aorta is then at least partially ligated forcing blood flow through the newly formed SMV. Finally, the nerve lead and myocardial leads are connected to an R-wave synchronous pulse-train stimulator.
Alternatively, the vascular grafts may be provided with valves (such as prosthetic heart valves) to control the flow of blood through the LVAD. The use of valves may obviate the need to ligate the aorta.
SMVs made as described above have been demonstrated to generate significant increases in cardiac output in Beagle dogs for periods of longer than 19 months. While effectiveness in humans remains to be demonstrated, animal results thus far appear promising.
A fundamental disadvantage of the above procedure lies in the removal of the plastic mandrel from the muscle pouch, sewing the patch material to the pouch and anastomosis of the vascular grafts to the patch material immediately preceding activation of the SMV. The needle punctures immediately preceding exposure of the SMV and vascular grafts to blood pressure often result in suture line bleeding at the patch and vascular graft suture lines and may result in disruption of a suture line.